<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
   .box{
     height: 200px;
     background-color: skyblue;
     margin: 30px 0;
   }
   .content{
     overflow: hidden;
   }
   .left{
    height: 300px;
    background-color: red;
    overflow: hidden;
   }
   .right{
     background-color: green;
     width: 200px;
     height: 200px;
     float: left;
     /* 浮动会脱离文档流 */
   }
</style>
<body>
    <div class="content">
        <!-- 行成了一个bfc -->
        <div class="box"></div>
        <div class="box"></div>
    </div>
    <div class="content">
        <!-- 行成了一个bfc -->
        <div class="box"></div>
    </div>
    <div class="back_box">
       <div class="right"></div>
       <div class="left"></div>
    </div>
</body>
</html>